Auto merge of #21104 - wayling:master, r=jdm

"./mach env" cause unclear message.

We did't use "LD_LIBRARY_PATH" and no need display this.

<!-- Please describe your changes on the following line: -->
Remove the print message.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ X] `./mach build -d` does not report any errors
- [ X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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python/servo/bootstrap_commands.py

@@ -39,12 +39,12 @@ class MachCommands(CommandBase):
              category='bootstrap')
     def env(self):
         env = self.build_env()
-        print("export RUSTFLAGS=%s" % env["RUSTFLAGS"])
-        print("export PATH=%s" % env["PATH"])
+        print("export RUSTFLAGS=%s" % env.get("RUSTFLAGS", ""))
+        print("export PATH=%s" % env.get("PATH", ""))
         if sys.platform == "darwin":
-            print("export DYLD_LIBRARY_PATH=%s" % env["DYLD_LIBRARY_PATH"])
+            print("export DYLD_LIBRARY_PATH=%s" % env.get("DYLD_LIBRARY_PATH", ""))
         else:
-            print("export LD_LIBRARY_PATH=%s" % env["LD_LIBRARY_PATH"])
+            print("export LD_LIBRARY_PATH=%s" % env.get("LD_LIBRARY_PATH", ""))
 
     @Command('bootstrap',
              description='Install required packages for building.',